This invention relates to the field of opthalmic lenses. More particularly, this invention relates to apparatus and a method for use in blocking lenses for machine processing of lenses.
Presbyopia is the loss of accommodation of the eye with age. It is classically treated by means of bifocal or trifocal spectacle lenses. Progressive lenses are the modern alternative to bifocal and trifocal lenses. Progressive lenses provide a continuous gradient of optical powers, ranging from the power required for viewing distant objects to that required for near objects. Because of the unbroken visual field provided by such lenses, many patients consider progressive lenses to be optically superior to the bifocal or trifocal lenses. Moreover, because of the absence of visible dividing lines, progressive lenses offer a distinct cosmetic advantage over conventional multifocals.
The progressive surface of a progressive lens ordinarily comprises the convex side of the lens. The sphero-cylindrical surface required to accommodate the patient's prescription is ground into the concave side of the lens. The generating, grinding, and polishing of the concave surface is a standard processing operation of the ophthalmic prescription laboratory.
Before the concave side of a lens can be surfaced, the lens must undergo a "blocking" operation. The operation consists of mounting a molded metal disk, or "block", on the convex side of the lens. The block provides an accurate mechanical reference surface for holding the lens in the chuck of the lens curve generator.
Typically the block consists of a low melting point alloy which is molded to shape directly against the convex surface of the lens to be blocked. The molding operation requires that the lens be held accurately against a reference ring, inside of which the melted alloy is poured and eventually solidifies.
Geometrically, the reference ring is a hollow right circular cylinder. When a progressive lens is blocked, the lens is placed against the blocking ring in such a way that only the distance portion of the lens actually makes contact with the inside edge of the ring. The reading half of the lens does not make contact with the ring. If the distance portion of the progressive side of the lens is essentially spherical, then accurate seating of the lens against the blocking ring is assured, and the lens may be blocked exactly as if the lens were an ordinary bifocal or trifocal. If, however, the distance Portion is aspherical, as is the case in the most advanced progressive-lens designs, then the lens may rock when placed against the blocking ring, and accurate seating against the ring becomes problematical. This can result in improper blocking and, subsequently, an improper generating operation leading to unwanted prism at the distance fitting center.
Invariably, when the distance portion of a progressive lens is aspherical, the asphericity is such that the average curvature in a horizontal merdian through the distance fitting center is greater than the average curvature in the distance portion half of a vertical merdian through the fitting center. That is, the distance portion of the progressive surface is slightly egg-shaped, the axis of the egg being vertical. It is clear that a surface of this shape cannot be mated to the end of a hollow right circular cylinder without rocking.
To achieve non-rocking contact it is possible, and has been proposed, to grind the end of the blocking ring to such shape as to mate exactly with the lens surface. This approach, however, is impractical and expensive in that it requires a large number of specially-shaped blocking rings to accommodate the various base curves and addition values associated with the complete progressive lens prescription range. Alternatively, a special blocking ring with three raised points on its perimeter could be used to provide three-point, non-rocking contact with the lens. These solutions, while technically feasible, are not very practical for the prescription laboratory, because the blocking apparatus is normally used for all lenses, and cannot be solely dedicated to the blocking of progressive lenses with aspherical distance portions.